Communications systems, and in general many other types of electronic systems, rely on a backplane based architecture comprising a plurality of circuit cards that plug into, or are otherwise electrically connected to, a system backplane. The system backplane is a facility over which the circuit cards can communicate with each other and over which power is supplied to the circuit cards. Examples of the type circuit cards included in communications systems include switch fabric cards, control cards, input/output (I/O) cards, line cards, and processor cards. Due to the need for high fault tolerance in such systems, also referred to as high availability, one of more of these types of circuit cards may be replicated in the system for redundancy.
A system backplane includes a backplane bus for inter communication between the circuit cards. A backplane bus commonly used for high speed systems is a Gunning Transceiver Logic (GTL) type bus. A GTL bus includes an open drain MOSFET transmitter circuit for each bit or signal line of the bus and one or more respective receiver circuits. A transmitter and receiver circuit may be included in one device thereby forming a GTL transceiver. The GTL bus has a low voltage swing, typically 0.8 volts, resulting in good performance at high data rates, low power consumption and minimal crosstalk between bus signals. Each bit line of a GTL bus is terminated to a termination voltage Vtt through a termination resistor Rtt, which is typically about 50 ohms. The termination voltage Vtt is typically 1.2 volts, although a newer version of the GTL specifications, GTL+, specifies a termination voltage of 1.5 volts. Herein after, the term GTL includes all such versions of the GTL buses unless stated otherwise. Additional details regarding GTL circuits and transceivers may be found by referring to U.S. Pat. No. 5,023,488 to Gunning et al.
In the case of redundant systems, the transmitter of a GTL bus may be replicated on another circuit card for higher system reliability. FIG. 1 depicts such a redundant GTL backplane bus as known in the prior art. Referring to FIG. 1, a redundant GTL backplane bus 10 includes a pair of termination cards 12, 14 labeled termination card A and B, that are both capable of driving electrical signals onto backplane connections 16 of the bus 10. However, only one of the termination cards 12, 14 drives the bus at any given time, that card being designated as an active card of the pair of termination cards, while the other termination card 12, 14 remains in a standby mode and is designated as an inactive card of the pair. The termination card A 12 has a plurality of GTL transmitters 18 each of which is connected to one of the backplane connections 16. Additionally, each of the GTL transmitters 18 is coupled at its output to a positive termination voltage VttA via a corresponding one of a plurality of pull-up resistors 20. The positive termination voltage VttA is generated locally on the termination card A by a low dropout (LDO) voltage regulator 22. Likewise, the termination card B 14 includes a plurality of GTL transmitters 24 each of which is connected to one of the backplane connections 16. Additionally, each of the GTL transmitters 24 on the termination card B is coupled at its output to a positive termination voltage VttB via a corresponding one of a plurality of pull-up resistors 26. The positive termination voltage VttB is generated locally on the termination card B by a low dropout (LDO) voltage regulator 28. The positive termination voltages VttA and VttB are intended to be equal in value, and are also referred to herein as Vtt when specificity to a particular termination card A or B is not required.
The redundant GTL backplane bus 10 may experience a problem if either one of termination cards A or B 12, 14 experiences an open or short circuit failure during normal operation. For example, if either one of the LDO voltage regulators 22, 28 or their associated components failed such that one of the termination voltages VttA or VttB was at ground potential. In that case, rather than having a bus terminated to equal positive termination voltages VttA and VttB via parallel termination at both ends of the backplane connections 16, each end would be terminated to a different voltage. This would likely cause the voltage levels on the bus 10 to be too low for successful data communications over the bus 10.
The type of failure just described with respect to the positive termination voltages VttA and VttB could be caused by various conditions such as component shorts/opens due to component aging or general malfunction of the card. However, another cause of particular concern is insertion of the inactive termination card. When the inactive termination card is inserted, a brief period of time passes before the LDO voltage regulator of that card can provide the termination voltage Vtt at its operational level. During this period of time, the operational effect is the same as having a termination voltage Vtt failure, and consequently the GTL backplane bus 10 is unable to reliably carry data communications between the various circuit cards of the system.
In view of the foregoing, it would be desirable to provide some means of protecting the operation of the GTL backplane bus against failures of either of the termination cards A or B, particularly with respect to the supply of the positive termination voltage Vtt.